1. Field of the Invention
This invention relates to an electromagnetic coil assembly, such as an electromagnetic clutch or brake for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning system, and more particularly, to the mounting structure for a thermal protection device in an electromagnetic apparatus.
2. Description of the Prior Art
An electromagnetic coil assembly with a thermal protection device is disclosed in JP 56-138529 to Teshima. In general, the electromagnetic clutch is provided with a thermal protection device, which is sensitive to abnormal temperature increases of the clutch. The thermal protection device shuts off the power source when abnormal frictional heat is generated due to slippage between the rotor and armature plate.
Referring to FIG. 1, an electromagnetic coil assembly comprises a bobbin 41 molded from plastic. Bobbin 41 includes a central tubular spool 41a and a pair of annular flanges 41b, 41c which project radially from the ends of spool 41a. A coil 44 is supported by bobbin 41 and is formed by multiple turns of electric wire 43 wound on spool 41a between flanges 41b, 41c. Thermal protection device 47, which has a pair of leads 45, 46 extending from both ends, is disposed on the outer surface of flange 41b. One end of lead 45 of thermal protection device 47 is secured to lead 50, which is connected to the power source (not shown), through caulking terminal 48. The other end of lead 46 of thermal protection device 47 is connected to electric wire 43 through caulking terminal 49.
In assembly, bobbin 41 is inserted into the inner portion of the housing, which has a U-shaped cross section, so that the outer end of flange 41b faces towards the opening in the housing. Thereafter, the cavity defined by bobbin 41, the housing and outer surface 41b are filled with a heated epoxy resin. As the resin hardens, coil 44 is fixed within the cavity.
Thermal protection device 47 is disposed adjacent to the open end of the cavity so that it is responsive to any increase in operating temperature. However, at the same time, thermal protection device 47 must be securely positioned against movement into the clutch rotor, since the clutch rotor is located adjacent the end surface 41b of bobbin 41. Thermal protection device 47 is secured to end surface 41b of bobbin 41 with electrical insulating tape 51, which is generally made of a resin, such a polyester resin or polyethylene terephthalate. Leads 45, 46 of thermal protection device 47 are made of electrically conductive material. Tape 51 insulates leads 45, 46 from clutch rotor.
Electrical insulating tape 51 generally has a smaller thermal conductivity than electrical epoxy resin. Consequently, when thermal protection device 47 is covered with electrical insulating tape 51 in addition to epoxy resin, the responsiveness of device 47 is reduced proportionately by the thermal conductivity of tape 51. In other words, excessive heat indicative of clutch failure will not reach thermal protection device 47 as quickly due to the presence of electrical insulating tape 51. Furthermore, in the assembly process, it is necessary to fixedly secure thermal protection device 47 on the end surface 41b of bobbin 41 with electrical insulating tape 51. The additional time needed to manipulate bobbin 41 and electrical insulating tape 51 so that thermal protection device 47 is properly positioned and secured is time consuming and reduces the efficiency of the assembly line.
These and other shortcomings of prior art electromagnetic coils are addressed by the electromagnetic coil of the preferred embodiments.